1. Field of the Invention
This invention relates to a method for recording an optical recording disc of phase change type.
2. Discussion of the Background
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording media is a phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline states whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge--Te system and Ge--Sb--Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe.sub.2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
In the phase change optical recording medium, grooves for tracking are formed on the substrate, and addressing information is often stored in the grooves. Recently, land/groove recording capable of high-density recording have gone into actual use, and in such land/groove recording, both the grooves and the lands between two adjacent grooves are used for the recording tracks. For example, DVD-RAM adopts a land/groove recording system.
DVD-RAM of a land/groove recording system, however, is incompatible with conventional read-only optical disc (DVD-ROM), and a countermeasure had to be taken on the side of the drive to enable the reproduction on both the DVD-RAM and the DVD-ROM. In other words, the DVD-RAM can not be reproduced on a player specifically designed for the DVD-ROM.
To produce an optical recording disc which can be reproduced on the player specifically designed for the DVD-ROM, groove recording is generally employed although land recording is also adoptable. In either case, use of either one of the groove and the land for the recording track is required.
When a recording density higher than that of the DVD-ROM (recording capacity, 4.7 GB) is to be realized by a medium of groove recording or land recording, width of the recording track (groove width or land width) should be reduced to an extremity, and it will be difficult to ensure sufficient degree of modulation and to minimize the jitter.